1. Field of the Invention
The present invention relates to a transmitting apparatus. More specifically, the present invention relates to a transmitting apparatus for bi-directional microwave communication from general households or small-scale offices to broadcast satellites or communication satellites using microwaves.
2. Description of the Background Art
The market for radio communication utilizing microwaves has been recently developed dramatically, along with developments of various systems including broadcast satellites and communication satellites. At the same time, the Internet has been developed and digital satellite broadcast has started, ever increasing the demand for bi-directional communication.
For bi-directional communication of a small-scale office or a general home using a broadcasting satellite or a communication satellite, it is a dominant practice at present to use the satellite broadcast as a signal transmission path (downstream) from the broadcasting station to a general home, and to use a telephone line as a signal transmission path (upstream) from a general home to the broadcasting station.
The telephone line used for the upstream supports only a slow rate of communication, and therefore it is not suitable for exchanging motion picture, for example, hindering promotion of satellite multimedia applications. Thus, there have been attempts to introduce satellite communication also to the upstream transmission, to enable bi-directional communication.
FIG. 6 shows a concept of bi-directional communication between each home and the broadcasting station through satellite broadcast. Referring to FIG. 6, a parabola antenna 51 is provided on the roof, for example, of a broadcasting station 50, and parabola antennas 62 and 63 are provided on the roofs of homes 60 and 61, respectively. Through broadcasting satellite 70, microwave bi-directional communication is performed between each of the parabola antennas 62 and 63 of respective homes and parabola antenna 51 of broadcasting station 50. For bi-directional communication, microwave of 12 GHz band is used for one direction and microwave of 14 GHz band is used for the other direction. An LNB (Low Noise Blocked Down Converter) similar to the one used in the conventional system for satellite broadcast reception is used as a receiver, and a transmitter is newly provided.
FIG. 7 is a block diagram representing a main portion of such a transmitter. The transmitter shown in FIG. 7 is positioned close to a parabola antenna provided outdoors of a household. An indoor unit, not shown, is provided indoors, by which a signal or an image input through a terminal apparatus such as a personal computer is converted to an intermediate frequency signal of 1 GHz, for example, superposed on a DC voltage of 12V, for example and transmitted to the transmitter through a coaxial cable.
In the transmitter shown in FIG. 7, the DC voltage input through a cable to an input terminal from an IDU (indoor unit) 30 and the intermediate frequency signal superposed thereon are separated by a capacitor C1 and an inductor L1. The intermediate frequency is input through capacitor C1 to a transmission circuit unit 1, and the DC voltage is input through inductor L1 to a power supply circuit unit 2. The intermediate frequency signal input to transmission circuit unit is subjected to amplification of the intermediate signal, frequency conversion from the intermediate frequency signal to a microwave signal having higher frequency based on a local oscillation signal from a local oscillation circuit 20 and amplification of the microwave signal, by an IF amplifier, a mixer 4 and a microwave amplifier 5 contained in transmission circuit unit 1, respectively. The microwave signal is input to a power amplifier 6 to be further amplified to a high power signal, and output from the transmitter, not shown.
Meanwhile, the DC voltage is input to power supply circuit unit 2, at which an optimal voltage to be supplied to transmission circuit unit 1 is generated. For example, when the DC voltage value applied to the input terminal is 13 to 26V, voltages of 8V, 5V and −5V are generated at power supply circuit unit 2, which are supplied to transmission circuit unit 1, respectively.
In the conventional circuit configuration shown in FIG. 7, however, there is a problem that even when a voltage lower than an operational DC voltage is applied, power supply circuit portion 2 operates, and supplies power to transmission circuit unit 1. The DC voltage supplied to the input terminal may possibly be lower than the operational voltage when IDU 30 inhibits operation of the transmitter, or when DC resistance increases because of an accidental bent of a cable connecting IDU 30 with the transmitter.
The voltage supplied from power supply circuit unit 2 to transmission circuit unit 1 depends on an input/output minimum voltage difference (for example, 2V) of a regulator in power supply circuit unit 2, and therefore, when the supplied voltage decreases to 7V, for example, voltage values of 6V, 4V and −4V, which are different from the values 8V, 5V and −5V in a normal operation, will be output.
At this time, it is possible that an active element malfunctions in the transmission circuit unit 1. For example, a negative voltage for gate bias of power amplifier 6 decreases, causing problems such as increase in drain current and heat build up and hence thermal self destruction, variation of S parameter of an active element caused by variation in the supply voltage to the active element and associated abnormal oscillation.